The Veterans Health Administration (VHA) is the largest single provider of heath care to individuals with spinal cord injury (SCI) in the United States. For 2016, Paralyzed Veterans of America estimates that 43,000 veterans with SCI received care from VHA. The annual cost of that care is substantial. In 2015, the total costs during the first year following an SCI ranged from $520K to $1.1M; recurring costs ranged from $69K to $185K per patient per year. Lifetime costs also continue to increase as life expectancy post-SCI increases. A key contributor to the high medical cost post-SCI is fragility fracture, often requiring prolonged hospitalization and specialized care. Up to three-quarters of individuals with SCI will sustain a fragility fracture in their lifetime. Fractures lead to serious medical complications, a loss of independence, and a loss of productivity, all resulting in substantial direct and indirect costs. SCI clinicians and patients agree that maintaining an active lifestyle is critical not only for general health, but also for musculoskeletal health. Given the substantial loss of bone that occurs in the lower limbs following SCI, however, clinicians must always be cognizant of the possibility of fracture, especially for those with more chronic injuries. Bone mineral density (BMD) measurement from a Dual-energy X-ray Absorptiometry (DXA) scan is the clinical gold standard for osteoporosis assessment in able-bodied individuals. Accurate diagnosis is important since it guides treatment and it helps to inform patients and doctors what activities can and can't be performed safely. Unlike the case with able-bodied individuals, there is no clinical standard and no consensus for assessing skeletal health in the lower limbs of individuals with SCI. For able-bodied individuals, the standard sites for scanning are the spine and hip, which are common sites of fracture in those with age-related osteoporosis. For those with SCI, however, most fractures occur just above and just below the knee. Hip and spine BMD are not good predictors of fracture at distal sites in the legs and, at present, no standardized protocols exist for assessing skeletal health near the knee. SCI clinicians continue to be faced with a critical question which is: Is it safe for my patient to participate in certain rehabilitation activities, recreation and sports activities, or to use an exoskeleton for ambulation? Currently, there is no evidence-based answer to that question. The purpose of this project is to validate scan protocols for bone mineral density assessment that will enable clinicians to address that issue. This in turn will allow clinicians to prescribe and monitor rehabilitation therapies and recreational activities that are appropriate for a particular patient given his or her skeletal heath. Over the past three decades more than a dozen different protocols have been proposed for scanning the area above and below the knee in patients with SCI, including nine protocols introduced since 2005. Those protocols have not been comprehensively assessed or compared for sensitivity or precision, nor have normative, reference values been determined for able-bodied individuals. Our study has four Specific Aims. Aim 1 is to generate normative databases for the multiple DXA protocols that have been proposed for bone density scanning of the distal femur and proximal tibia. Aim 2 is to determine and rank the precision of those protocols in able-bodied individuals. Aim 3 is to the determine precision for the knee DXA protocols examined in Aim 1, but in individuals with SCI >4 years post-injury, and to see if High-Resolution QCT and peripheral- QCT scanning provide clinically valuable complementary data compared to DXA. Aim 4 is to measure bone changes over time in patients 1 to 4 years post-injury and, for DXA, to compare those changes to the least detectable change determined from the precision for each candidate knee DXA protocol. The results of this study will have immediate clinical utility and will lay the groundwork for future development of a fracture risk assessment tool specific for persons with SCI, comparable to risk assessment tools already available for able- bodied individuals. Clinical implementation of validated DXA protocols will be the immediate next step.